The Reapers are a large group of sentient inorganic space ships. Their prime directive is to eradicate all advanced organic life in the galaxy as a kind of harvest to allow the more primitive species a chance to grow and build the galaxy's infrastructure back up only to have it wiped clean once again.

The horror of the reapers isn't that they just kill populations, they turn your own people against you by turning them into monsters or by controlling their minds and actions seemingly telepathically. They're not interested in synthetics, outside of those who wish to join them, like some of the geth, but they will stop at nothing to eradicate all advanced life.